Geared turbofan engine gearbox arrangement

ABSTRACT

A turbofan engine according to an exemplary aspect of the present disclosure includes, among other things, a fan having a plurality of blades, and a transmission is configured to drive the fan. The fan blades have a peak tip radius R T . The fan blades have an inboard leading edge radius R H  at an inboard boundary of the flowpath. A ratio of R H  to R T  is less than about 0.40.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application of Ser. No. 14/496,574, filed Sep.25, 2014, which is a divisional application of Ser. No. 14/087,471,filed Nov. 22, 2013, and entitled “Geared Turbofan Engine GearboxArrangement”, the disclosure of which is incorporated by referenceherein in its entirety as if set forth at length.

BACKGROUND

The disclosure relates to turbofan engines.

Gas turbine engines and similar structures feature a number ofsubassemblies mounted for rotation relative to a fixed case structure.Such engines typically have a number of main bearings reacting radialand/or thrust loads. Examples of such bearings are rolling elementbearings such as ball bearings and roller bearings. Typically suchbearings all react radial loads. Some such bearings also react axial(thrust) loads (either unidirectionally or bidirectionally). Ballbearings typically react thrust loads bidirectionally. However, if theinner race is configured to engage just one longitudinal side of theballs while the outer race engages the other longitudinal side, the ballbearing will react thrust unidirectionally.

Tapered roller bearings typically react thrust unidirectionally. Twooppositely-directed tapered roller bearings may be paired or “duplexed”to react thrust bidirectionally. An example is found in the fan shaftbearings of U.S. Patent Application Publication 2011/0123326A1, which isincorporated herein by reference in its entirety and which is entitled“Bellows Preload and Centering Spring for a Fan Drive Gear System”.

U.S. Patent Application Publication 2013/0192198, which is incorporatedherein by reference in its entirety and which entitled “CompressorFlowpath”, discloses a flowpath through a compressor having a low slopeangle.

For controlling aspects of the flowpath passing through the fan duct,some turbofan engines include controllable features such as variable fanblade pitch and variable area fan exhaust nozzles. U.S. Pat. No.5,431,539, which is incorporated herein by reference in its entirety andwhich is entitled “Propeller Pitch Change Mechanism”, and U.S. Pat. No.5,778,659, which is incorporated herein by reference in its entirety andwhich is entitled “Variable Area Fan Exhaust Nozzle Having MechanicallySeparate Sleeve and Thrust Reverser Actuation System”, discloserespective such systems.

Unless explicitly or implicitly indicated otherwise, the term “bearing”designates an entire bearing system (e.g., inner race, outer race and acircumferential array of rolling elements) rather than the individualrolling elements. The term “main bearing” designates a bearing used in agas turbine engine to support the primary rotating structures within theengine that produce thrust. This is distinguished, for example, from anaccessory bearing (which is a bearing that supports rotating structuresthat do not produce thrust such as the fuel pump or oil pump bearings inan accessory gearbox).

There are several different factors influencing flowpath geometry atcertain locations in the engine. Weight, material strength andaerodynamics influence desirable core flowpath radius at differentlocations within the compressor and turbine sections. As noted above,U.S. Patent Application Publication 2013/0192198 discloses certainadvantageous aspects of flowpath geometry within a compressor. This,however, may be competing with considerations regarding the coreflowpath elsewhere in the engine. For example, the presence of anactuation mechanism or variable pitch fan blades may mandate arelatively large hub diameter. Similarly, the presence of a drive gearsystem axially between the compressor and the fan may also causerelatively high core flowpath diameters. Normally, it may be desirableto minimize radial turning of the core flow between such high radiussections and a lower diameter compressor section downstream thereof. Ofparticular importance to flowpath geometry and overall engineefficiency, however, are the bearing arrangements used to support thevarious rotating structures; improvements in this area are, therefore,always of interest to the turbofan engine designer.

SUMMARY

One aspect of the disclosure involves a three-spool turbofan enginecomprising a fan having a plurality of blades. A transmission isconfigured to drive the fan. The fan blades have a peak tip radiusR_(T). The fan blades have an inboard leading edge radius R_(H) at aninboard boundary of the flowpath. A ratio of R_(H) to R_(T) is less thanabout 0.40.

A further embodiment may additionally and/or alternatively include theengine being a three-spool turbofan engine comprising a first spoolcomprising a first pressure turbine and a first shaft coupling the firstpressure turbine to the transmission. A second spool comprises a secondpressure turbine, a first compressor, and a second spool shaft couplingthe second pressure turbine to the second spool compressor. A core spoolcomprises a third pressure turbine, a second compressor, and a coreshaft coupling the third pressure turbine to the second compressor. Acombustor is between the second compressor and the third pressureturbine.

A further embodiment may additionally and/or alternatively include thefirst compressor having a rear hub engaging a bearing, said bearingengaging the first shaft, and another bearing engaging the firstcompressor and the first shaft.

A further embodiment may additionally and/or alternatively include aring gear of the transmission being mounted to rotate with the fan as aunit.

A further embodiment may additionally and/or alternatively include eachfan blade having a leading edge and a trailing edge. A splitter ispositioned along a flowpath through the engine and having a leading rimseparating a core branch of the flowpath from a bypass branch of theflowpath. An inboard boundary of the core flowpath has a radius R_(II)at an axial position of the splitter rim and a radius R_(I) at a leadingstage of blades of the first compressor. A ratio of an axial length L₁₀between the splitter rim and the leading stage of blades of the firstcompressor at the inboard boundary of the core flowpath to the radiusR_(II) is less than 1.2.

A further embodiment may additionally and/or alternatively include eachfan blade having a leading edge and a trailing edge. A splitter ispositioned along a flowpath through the engine and having a leading rimseparating a core branch of the flowpath from a bypass branch of theflowpath. An inboard boundary of the core flowpath has a radius R_(II)at an axial position of the splitter rim and a radius R_(I) at a leadingstage of blades of the first compressor. A ratio of the radius R_(I) tothe radius R_(II) is greater than 0.50. The ratio of the radius R_(I) tothe radius R_(II) may be 0.55-1.0.

A further embodiment may additionally and/or alternatively include thefan blades being non-variable.

A further embodiment may additionally and/or alternatively include avariable fan nozzle.

A further embodiment may additionally and/or alternatively include theengine having a plurality of main bearings. A first of said mainbearings engages a static support and a forward hub of the second spool.A second of said main bearings engages the first shaft and the forwardhub of the second spool.

A further embodiment may additionally and/or alternatively include thefirst bearing and the second bearing be behind the transmission.

A further embodiment may additionally and/or alternatively include alength between said first of said main bearings and a center of gravityof said rotor of the first compressor being less than half of adisk-to-disk overall length of the first compressor.

A further embodiment may additionally and/or alternatively include alength between said first of said main bearings and a center of gravityof a rotor of the second spool compressor being less than a radius R_(I)of the inboard boundary of the core flowpath at a leading stage ofblades of the first compressor.

A further embodiment may additionally and/or alternatively include theforward hub extending forward from a disk of the first compressor.

A further embodiment may additionally and/or alternatively include theforward hub extending forward from a bore of the disk of the firstcompressor.

A further embodiment may additionally and/or alternatively include thefirst compressor having at least one disk forward of said disk.

A further embodiment may additionally and/or alternatively include thestatic support passing through said at least one disk forward of saiddisk.

A further embodiment may additionally and/or alternatively include thefirst compressor having at least two disks forward of said disk.

A further embodiment may additionally and/or alternatively include saidat least one disk being forward of a centerplane of the second bearing.

A further embodiment may additionally and/or alternatively include thefirst bearing and the second bearing being non-thrust roller bearings.

A further embodiment may additionally and/or alternatively includerollers of the first bearing and the second bearing being at leastpartially longitudinally overlapping.

A further embodiment may additionally and/or alternatively include aseparation of a transverse centerplane of the first bearing and atransverse centerplane of the second bearing being less than a radius(R_(B)) of the first bearing.

A further embodiment may additionally and/or alternatively include afirst seal sealing the first bearing and a second seal sealing thesecond bearing to isolate a transmission compartment ahead of the firstbearing and the second bearing from a region behind the first bearingand the second bearing.

A further embodiment may additionally and/or alternatively include thetransmission comprising: a sun gear mounted to rotate with the firstshaft; a ring gear mounted to rotate with the fan; a plurality ofintermediate gears between the sun gear and the ring gear; and a carrierholding the intermediate gears.

A further embodiment may additionally and/or alternatively include athird of said main bearings being a thrust bearing engaging the firstspool shaft.

A further embodiment may additionally and/or alternatively include afourth of said main bearings being a non-thrust roller bearings bearingengaging an aft end of the first spool shaft.

A further embodiment may additionally and/or alternatively include thecore shaft engaging at least two of said main bearings, and wherein atleast one of said at least two of said main bearings is a thrustbearing.

A further embodiment may additionally and/or alternatively include thefirst pressure turbine having three to five blade stages.

A further embodiment may additionally and/or alternatively include thesecond spool shaft engaging at least two of said main bearings, at leastone of which is a thrust bearing.

A further embodiment may additionally and/or alternatively include aninter-shaft bearing axially locating the first spool shaft.

A further embodiment may additionally and/or alternatively include thefirst spool shaft engaging at least three of said main bearings.

A further embodiment may additionally and/or alternatively include thefan being a single-stage fan.

A further embodiment may additionally and/or alternatively include theratio of R_(H) to R_(T) being less than about 0.30.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic longitudinal sectional view of a first turbofanengine embodiment.

FIG. 1A is an enlarged view of a forward portion of the engine of FIG.1.

FIG. 2 is a schematic longitudinal sectional view of a second turbofanengine embodiment.

FIG. 2A is an enlarged view of a forward portion of the engine of FIG.2.

FIG. 3 is a schematic longitudinal sectional view of a third turbofanengine embodiment.

FIG. 4 is a schematic longitudinal sectional view of a fourth turbofanengine embodiment.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIG. 1 shows a turbofan engine 20 having a central longitudinal axis orcenterline 500. The engine has a structural case including a core case22. The exemplary structural case further comprises a fan case 24connected to the core case by a circumferential array of struts 26 andsurrounding a fan 28. The core case and the fan case may have respectiveoutboard aerodynamic nacelles (shown schematically as 31 and 32).

The exemplary forward rim of the fan case is proximate an engine inlet30 receiving an inlet flow 502 when the engine is operating. The inletflow passes downstream through the fan 28 and divides into a core flow504 passing inboard along a core flowpath 506 (core branch of a combinedflowpath) within the core case and a bypass flow 508 passing outboardalong a bypass flowpath 510 (bypass branch of a combined flowpath)between the core case 22 and the fan case 24.

The bypass flowpath extends to an outlet 38. The exemplary outlet 38 isdefined by a variable nozzle assembly 35. The exemplary variable nozzleassembly 35 includes a movable member 36 having a downstream/trailingend 37 for defining the outlet between the end 37 and the core nacelle31. The exemplary member 36 may articulate between at least twoconditions or positions. The exemplary articulation involves an axialtranslation between a forward/retracted condition or position shown insolid line and a rearward/extended condition or position shown in brokenline with the numeral 36′. The translation may be driven by an actuator(not shown) (e.g., a hydraulic actuator).

The core flow 504 (or a majority portion thereof allowing for bleeds,etc.) passes sequentially through one or more compressor sections, acombustor, and one or more turbine sections before exiting a core outlet34. In the exemplary engine the fan is a single-stage fan having asingle stage of fan blades 40. Each of the compressor and turbinesections may include one or more blade stages mounted to rotate as aunit about the centerline 500. The blade stages may be alternatinglyinterspersed with vane stages. Each compressor section is co-spooledwith an associated turbine section. From upstream to downstream alongthe core flowpath, the exemplary engine has two compressor sections 42and 44, the combustor 45, and three turbine sections 46, 48, and 50. Thefan and compressor sections (and their stages) progressively compressinlet air which passes into the combustor for combustion with fuel togenerate gas of increased pressure which passes downstream through theturbine sections where the gas pressure is progressively reduced as workis extracted. The turbine section 46 operates at a pressure that ishigher than the intermediate turbine 48 and the low turbine 50 and isoften referred to as a high (or third) pressure turbine (HPT) or a coreturbine. The HPT blade stages are connected via a shaft 52 (“high shaft”or “core shaft”) to the blade stages of the compressor section 44 todrive that compressor section (often referred to as a high pressurecompressor (HPC) or core compressor) to form a high spool or core spool.

The turbine section 48 operates at a pressure range that is intermediateto the low and high pressure sections 50 and 46. The turbine section 48is thus often referred to as an intermediate (or second) pressureturbine (IPT). The IPT blade stages are connected via a shaft 54(“intermediate shaft”) to the compressor section 42 to drive thatcompressor section (often referred to as an intermediate pressurecompressor (IPC)) to form an intermediate spool.

The turbine section 50 operates at a low pressure range relative to thehigh pressure turbine 46 and the intermediate pressure turbine 48 and isthus often referred to as a low (or first) pressure turbine (LPT) or asa fan drive turbine. The LPT blade stages are connected via a shaft 56(“low shaft”) to a transmission 60 (e.g., an epicyclic transmission,more particularly a geared system known as a fan drive gear system(FDGS)) to indirectly drive the fan 28 with a speed reduction.

An exemplary high pressure turbine 46 is a single or double stageturbine assembly (although three or more HPT stages are possible); anexemplary intermediate stage turbine 48 is a single or double stageturbine assembly (although three or more IPT stages are possible); anexemplary low pressure turbine 50 is a multi-stage turbine such as, forexample, one or more stages, or more specifically three to five stages(although one or two stages is also possible).

The exemplary transmission 60 (FIG. 1A) comprises a centralexternally-toothed sun gear 80. The sun gear 80 is encircled by aninternally-toothed ring gear 82. A number of externally-toothed star orplanet gears 84 are positioned between and enmeshed with the sun gear 80and ring gear 82. The star or planet gears 84 can be referred to asintermediate gears. A cage or carrier assembly 86 carries theintermediate gears via associated bearings 88 for rotation aboutrespective bearing axes. The exemplary bearings 88 may be rollingelement bearings (e.g., ball or roller bearings) or may be journalbearings having external circumferential surface portions closelyaccommodated within internal bore surfaces of the associatedintermediate gears 84. Regardless of the type, the bearings may bemetallic (such as aluminum titanium, other metal, or an alloy of morethan one metal), ceramic, composite, or other material.

The exemplary carrier assembly 86 comprises a front plate (e.g.,annular) in front of the gears and a rear plate (e.g., annular) behindthe gears. These plates may be mechanically connected by the bearings 88and/or by linking portions between adjacent intermediate gears.

In the exemplary embodiment, a forward end of the low shaft 56 iscoupled to the sun gear 80. The exemplary low shaft 56 has a generallyrigid main portion 100 and a flexible forward portion 102. A forward endof the portion 102 may have a splined outer diameter (OD) surfaceinterfitting with a splined inner diameter (ID) surface of the sun gear80 to transmit rotation.

The exemplary carrier assembly 86 is substantially non-rotatably mountedrelative to the engine case 22. In the exemplary embodiment, the carrierassembly 86 is coupled to the case 22 via a compliant flexure 110 thatallows at least small temporary radial and axial excursions androtational excursions transverse to the centerline 500. The exemplaryflexure 110 carries a circumferential array of fingers 111 engaging thecarrier 86 (e.g., between adjacent gears 84). A peripheral portion ofthe flexure 110 is mounted to the case to resist rotation about thecenterline 500. Thus, flexing of the flexure accommodates the smallexcursions mentioned above while holding the carrier against rotationabout the centerline.

The exemplary ring 82 is coupled to the fan 28 to rotate with the fan 28as a unit. In the exemplary embodiment a rear hub 122 of a main fanshaft 120 connects the fan 28 to the ring gear 82.

The speed reduction ratio is determined by the ratio of diameters of thering gear 82 to the sun gear 80. This ratio will substantially determinethe maximum number of intermediate gears 84 in a given ring. The actualnumber of intermediate gears 84 will be determined by stability andstress/load sharing considerations. An exemplary reduction is betweenabout 2:1 and about 13:1. Although only one intermediate gear 84 isnecessary, in exemplary embodiments, the number of intermediate gears 84may be between about three and about eleven. An exemplary gear layoutwith fixed carrier is found in U.S. Patent Application Publication2012/0251306A1, which is incorporated by reference herein in itsentirety and which is entitled “Fan Rotor Support. In addition, althoughthe exemplary transmission 60 is described as being of a “star” typeconfiguration, other types of configurations (such as “planetary”systems) are within the scope of this invention.

Thus, the exemplary engine 20 has four main rotating components (units)rotating about the centerline 500: the core spool (including the highpressure turbine 46, the high shaft 52, and the high pressure compressor44); the intermediate spool (including the intermediate pressure turbine48, the intermediate shaft 54, and the intermediate pressure compressor42); the low spool (including the low pressure turbine 50, low shaft 56,and the sun gear 80); and the fan assembly (including the fan 28 itself,the fan shaft 120, and the ring gear 82). Each of these four thingsneeds to be supported against: radial movement; overturning rotationstransverse to the centerline 500; and thrust loads (parallel to thecenterline 500). Radial and overturning movements are prevented byproviding at least two main bearings engaging each of the four units.

Each unit would have to also engage at least one thrust bearing. Thenature of thrust loads applied to each unit will differ. Accordingly,the properties of the required thrust bearings may differ. For example,the fan 28 primarily experiences forward thrust and, therefore, thethrust bearings engaging the fan 28 may be configured to address forwardthrust but need not necessarily address rearward thrusts of similarmagnitudes, durations, etc.

The FIG. 1 embodiment has two main bearings 148, 150 along the fan shaftforward of the transmission 60. Inboard, the inner race of each bearing148, 150 engages a forward portion of the shaft 120 aft of the fan 28.Outboard, the outer race of each bearing 148, 150 engages staticstructure of the case. The exemplary static structure comprises asupport 152 extending inward and forward from a forward frame 154. Thesetwo bearings 148, 150 thus prevent radial excursions and overturningmoments which the fan 28 may produce during flight.

To resist thrust loads, one or both of the bearings 148, 150 may bethrust bearings. In an exemplary embodiment, both are thrust bearings(schematically shown as ball bearings). Both may be thrust bearingsbecause there may typically be no differential thermal loading (and thusthermal expansion) of the support 152 relative to the shaft 120 betweenthese bearings. Where the two coupled structures are subject todifferences in thermal expansion, it may be desirable to have only onebearing be a thrust bearing.

In one alternative example of a single thrust bearing and a singlenon-thrust bearing, the bearing 150 would be a straight roller bearingwith longitudinal roller axes configured to only handle radial loads.The other bearing (i.e., the bearing 148) would be a thrust bearing. Dueto the significance of forward thrust loads on the fan 28, the bearing148 may be biased to resist forward loads. The exemplary bearing 148 maythen be a bidirectional ball bearing or a bidirectional tapered rollerbearing (e.g., wherein the rollers have a forward taper and forwardlyconverging roller axes to preferentially handle the forward thrustloads). A similar bidirectional tapered roller bearing is shown in U.S.Pat. No. 6,464,401 of Allard, which is incorporated herein by referencein its entirety and which is entitled “High Load Capacity Bi-DirectionalTapered Roller Bearing”. Ball bearings are typically bidirectionalthrust bearings. However, a unidirectional ball bearing may be formed byhaving at least one of the races contacting only a single longitudinalside of the balls.

An exemplary bearing arrangement for supporting the remaining threeunits is discussed below. Various aspects of each of these may beindependently implemented or all may be implemented in a given engine.

The exemplary low shaft 56 is principally radially supported by aforward bearing 162, an intermediate bearing 170, and an aft bearing172. The exemplary forward bearing 162 is indirectly radially groundedto the case 22. An exemplary indirect grounding (discussed furtherbelow) is via the intermediate spool and bearing 160. The exemplarybearing 160 (FIG. 1A) is directly radially grounded to the case (e.g.,by a bearing support 164 extending inward from a frame 154 aft of thesupport 152). FIG. 1 also shows an inlet guide vane array 155immediately upstream of the struts of the frame 154 and an outlet guidevane array 157 immediately downstream of the frame 154 and upstream ofthe leading compressor stage. In exemplary implementations, the vanes ofthe array 157 may be variable vanes. The exemplary array 155 isimmediately downstream of a splitter 159 dividing the core flowpath fromthe bypass flowpath.

The exemplary bearing 170 intervenes directly between the low spool andintermediate spool at an intermediate location. In the exemplaryembodiment, it is indirectly radially grounded by the bearing 220. Thebearing 220 is directly radially grounded by a support 240 extendingradially inward from a structural vane array (frame) 242 between thecompressor sections 42 and 44.

The exemplary aft bearing 172 is directly radially grounded to the case22 via a support 180 extending inward from a frame 182 extending acrossthe core flowpath 504. The exemplary support 180 is aft of the LPT 50with the frame 182 being a turbine exhaust frame. Alternativeimplementations may shift the support 180 forward of the LPT 50 toengage an inter-turbine frame 183 between the turbine sections 48 and50.

In the exemplary embodiment, the bearings 162 and 172 are non-thrustroller bearings (e.g., straight roller bearings). The bearing 170 servesas inter-shaft thrust bearing (e.g., a bidirectional ball bearing)having an inner race engaging an intermediate portion of the low shaft56 and an outer race engaging the intermediate shaft 54 to indirectlyaxially ground the low shaft 56 to the case 22 via the intermediateshaft 54.

By locating the bearing 170 relatively axially close to the bearing 220,the bearing 170 may also provide an intermediate location of radialgrounding in addition to the forward and aft radial groundings providedby the bearings 162 and 172. Alternative implementations might eliminateor reduce the amount of this radial grounding. In the FIG. 1 example,the bearings 160 and 162 are stacked so close as to be partially axiallyoverlapping (i.e., axial overlap of their rollers) to provide a highdegree of radial support.

In contrast, there is a slight non-overlap forward shift of the bearing170 relative to the bearing 220. In the exemplary engine, the outer raceof the bearing 170 engages a forwardly-projecting support extendingforward from a rear hub 174 of the compressor section 42. The exemplaryrear hub 174 extends from a bore 175 of one of the disks of thecompressor section 42. Slight flexing of the hub 174 and the outerbearing support 173 protruding therefrom may provide a little moreradial compliance than associated with the forward bearing 162.

The intermediate spool is supported by forward bearing 160, intermediatebearing 220, and an aft bearing 230. In an exemplary embodiment, forwardbearing 160 is a non-thrust roller bearing providing radial retentiononly. The inner race of the bearing 160 (and outer race of the bearing162) are mounted along respective outer and inner faces of a hub orsupport 236 extending forward from the bore 237 of one of the disks ofthe compressor section 42 (e.g., the first (upstream-most) disk). Theexemplary intermediate bearing 220 is a bidirectional thrust bearing(e.g., ball bearing) directly radially and axially supporting/groundingthe intermediate spool via the support 240 extending to theinter-compressor frame 242 between the compressor sections 42 and 44.The bearing 230 indirectly radially supports/grounds the intermediatespool by engaging the intermediate spool and the low spool. In theexemplary embodiment, the inner race of the bearing 230 engages aportion of the intermediate shaft aft of the turbine section 48 and theouter race of the bearing 230 engages a support extending forward from ahub 248 of the LPT 50. The exemplary hub 248 extends forward from thebore of a disk (e.g., the last or downstream-most disk) of the LPT.

The radial loads on the intermediate spool at the bearing 230 willprimarily be transmitted to the low shaft 56 and through an aft portionof the low shaft 56 to the bearing 172 and grounded by the support 180and frame 182. Axial (thrust) loads will pass through the bearing 220.

Thus, thrust loads on the low spool are transmitted via the shaft 56through the bearing 170, through the intervening portion of theintermediate shaft/spool, to the bearing 220, and grounded back throughthe support 240.

The core spool may be fully directly supported by two bearings 250 and260 of which at least one would be a thrust bearing. In the exemplaryembodiment, the bearing 250 is a forward bearing grounding a forwardportion of the core shaft ahead of the compressor section 44 to theinter-compressor frame 242 via a support 270. The aft bearing 260grounds a portion of the core shaft intermediate the compressor section44 and turbine section 46 via a support 272 extending to a combustorframe 274 ahead of the turbine section 46. In alternative embodiments,this aft bearing 260 may be shifted aft of the turbine section 46 via asupport (not shown) to an inter-turbine frame 278 between the sections46 and 48. In the exemplary implementation, the bearing 250 is a thrustbearing (e.g., a bidirectional ball bearing with its inner race engagingthe core spool and its outer race engaging the support 270). Theexemplary bearing 260 is a straight roller bearing with its inner raceengaging the core shaft 52 and its outer race engaging the support 272.The exemplary support 270 extends to a rear portion of the frame 240 aftof the support 242. The exemplary inner race of the bearing 250 ismounted to a hub or support extending forward from a bore of a disk(e.g., the upstream-most disk) of the compressor section 44.

FIG. 1 further shows the transmission 60 as having a centerplane 516 andthe gears as having a gear width W_(G) and the fan blade array as havinga centerplane 518. From fore to aft, the bearings have respectivecenterplanes 520, 522, 524, 526, 528, 530, 532, 534, 536, and 538.

As discussed above, an exemplary embodiment places the centerplanes 524and 526 of the bearings 160 and 162 relatively close to each other so asto best transmit radial loads from the low shaft 56 to the case. Anexemplary separation between the planes 524 and 526 (FIG. 1A) in suchembodiments is less than the characteristic radius of the bearing 160(e.g., radius R_(B) relative to the axis 500 of the intersections of theindividual rolling element axes with the bearing centerplane). Incontrast, the exemplary embodiment has a greater separation between thecenterplanes 528 and 530 of the bearings 170 and 220. This may provide agreater radial compliance at the associated intermediate location.

FIG. 1A further shows a transmission compartment 286 containing thetransmission 60. Aftward, the transmission compartment is largelybounded by the support 164 and bearings 160 and 162. Seals may beprovided to seal the transmission compartment 286 from a region 288(e.g., a compressor compartment) aft thereof. The exemplary sealscomprise an outer seal 290 sealing between the static structure and theintermediate spool and an inner seal 292 sealing between theintermediate spool and the low spool. Exemplary seal 290 is held by acarrier 291. An exemplary carrier 291 is formed as an inward and aftwardextension of the support 164 holding the seal 290 in sliding/sealingengagement with the low spool (e.g., with an inner race of the bearing160). Similarly, a seal carrier 293 carries the exemplary seal 292. Inthe exemplary embodiment, the seal carrier 293 is mounted to or formedas a portion of the low shaft main portion 100 holding the seal 292 insealing and sliding engagement with the intermediate spool (e.g., withan outer race of the bearing 162). In alternative implementations, thecarrier and seal elements of one or both of the sealing systems may bereversed (e.g., the seal carrier 293 could be formed as a portion of thehub 236 holding the seal 292 in sliding/sealing engagement with the lowspool).

FIG. 2 shows an alternate embodiment 320 which may be otherwise similarto the engine 20 but which has a forward shift of its compressor section42′ relative to the compressor section 42 of FIG. 1. The exemplaryforward shift may be achieved by having the hub or support structure 236(FIG. 2A) that cooperates with the bearings 160 and 162 extend forwardfrom the bore 237′ of an intermediate disk of the compressor section 42′in distinction to the extension from the upstream-most disk of thecompressor section 42. In the exemplary engine 320, the hub 236 (FIG.2A) extends from the third disk leaving two disks and their associatedblade stages thereahead. The exemplary shift shifts at least one diskstage forward of the bearings 160 and/or 162. In this example, thelongitudinal position of the first disk (e.g., measured by thecenterplane of its web and/or bore) is shifted ahead of the centerplanesof the bearings 160 and 162. An exemplary shift places the first diskahead of both bearings 160 and 162 and the second disk ahead of only thebearing 162. However, other locations and combinations are possible.

A further characterization of the longitudinal compactness involves therelationship between the first disk and the transmission. FIG. 2A showsa centerplane 560 of the first disk 340. The centerplane 560 is behindthe gear centerplane 516 by a length L_(D). Exemplary L_(D) is 2.0 timesthe gear width W_(G) or less, more particularly, 1.5 times W_(G) orless. Alternatively characterized, exemplary L_(D) is 60% or less of thecore flowpath inboard radius R_(I) at the disk centerplane 560, moreparticularly, 50% or less or 35% or less of R_(I).

Yet alternatively characterized relative to such a core flowpath inboardradius R_(G) at the gear centerplane 516, exemplary L_(D) is 50% ofR_(G) or less, more particularly, 40% or less or 30% or less.

To further facilitate longitudinal compactness, relative to the engine20, the engine 320 axially shrinks the frame 154′ relative to the frame150. In this example, the frame 154′ and its associated struts replaceboth the frame 154 and its associated struts and the inlet guide vanearray 155 (FIG. 1A). The guide vane array 157 (FIG. 1A) downstream ofthe struts is effectively shifted forward to become 157′. Along with theforeshortening of the frame 154′, the outboard periphery and mountinglocation of the support 164 is shifted forward and outward to become164′. Thus, the exemplary support 164′ is shallower than support 164 andpartially overarches the span of the transmission gears. Because of thisoverarching, the fingered flexure 110 is shifted to be mounted to amounting feature (e.g., flange) 110′ along the support 164′.

FIG. 3 shows yet a further embodiment 420 reflecting the variationdiscussed above wherein the bearing 260 is shifted aft of the highpressure turbine section 46. Other variations might add a secondintermediate spool. Other variations include unducted fans. Othervariations include multi-stage fans. FIG. 4 shows an axially compactengine lacking the forward shift of the FIG. 3 embodiment but, instead,being otherwise similar to FIG. 1 from the leading compressor diskforward. LPC stage count, however, is reduced relative to the FIG. 1embodiment so that the rear end of the LPC is shifted forward relativeto the FIG. 1 embodiment as is the rear end of the LPC of the FIG. 3embodiment.

FIG. 4 further shows various radial measurements. A fan tip radius isshown as R_(T). In this exemplary embodiment, the tip maximum radius isat the blade leading edge. FIG. 4 further shows a characteristic hubradius R_(H). Exemplary R_(H) is defined as the flowpath inboard orinner diameter (ID) radius at the fan blade leading edge. FIG. 4 furthershows a core inlet inner radius R_(II) which is measured as the inboardor ID radius of the core flowpath at the axial position of the forwardrim of the splitter 159. A characteristic compressor inlet radius may bemeasured as the aforementioned R_(I). Alternatively, this radius may bemeasured at the leading edge of the associated upstreammost blade stage.These will typically be very close to each other.

FIG. 4 further shows an axial length L₁₀ between the locations at whichR_(II) and R_(I) are measured. FIGS. 2 and 4 also label a length L_(CG)between a centerplane of the bearing 160 which may represent the closestmain bearing behind the FDGS, the forwardmost bearing interveningdirectly (between the intermediate spool and the case or both) and thetransverse plane of the center of gravity of the intermediate pressurecompressor rotor (e.g., ignoring the intermediate pressure shaft aft ofthe bearing 170 and ignoring the intermediate pressure turbine rotor). Acharacteristic intermediate pressure compressor length L_(IC) is shownas the center-to-center axial distance between the leading and trailingdisks. Particularly, with the forward shifting of the IPC of theembodiments of FIGS. 2 and 3 but also with the foreshortening of theembodiment of FIG. 4, L_(CG) may be reduced relative to FIG. 1. Theexemplary L_(CG) may be reduced to less than R_(II) and even to lessthan R_(I). L_(CG) may also be reduced to less than one half of L_(IC)(e.g., as shown in FIGS. 2 and 3). The particular reconfiguration ofFIGS. 2 and 3 helps bring the center of gravity close to the plane ofthe bearing 160 to maintain stability. This stability reduces the radialloads that must be reacted by the bearing 220. Thus, the bearing 170 maybe more specifically configured for reacting thrust loads with lesscapacity to react radial loads and may be lightened.

The use of “first”, “second”, and the like in the following claims isfor differentiation within the claim only and does not necessarilyindicate relative or absolute importance or temporal order. Similarly,the identification in a claim of one element as “first” (or the like)does not preclude such “first” element from identifying an element thatis referred to as “second” (or the like) in another claim or in thedescription.

One or more embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made. For example, whenapplied to an existing basic configuration, details of suchconfiguration or its associated environment may influence details ofparticular implementations. Accordingly, other embodiments are withinthe scope of the following claims.

What is claimed is:
 1. A turbofan engine comprising: a fan having a plurality of blades rotatable about a longitudinal axis; a transmission configured to drive the fan; and a first spool and a second spool, wherein: the fan blades have a peak tip radius R_(T); the fan blades have an inboard leading edge radius R_(H) at an inboard boundary of a flowpath; a ratio of R_(H) to R_(T) is less than 0.40; said first spool comprises: a first pressure turbine; and a first spool shaft coupling the first pressure turbine to the transmission; said second spool comprises: a second pressure turbine; a first compressor; and a second spool shaft coupling the second pressure turbine to the first compressor; the engine comprises a plurality of main bearings wherein: a first of said main bearings engages a static support and a forward hub of the second spool; and a second of said main bearings engages the first spool shaft and the forward hub of the second spool; the fan is a single-stage fan; and a ring gear of the transmission is mounted to rotate with the fan as a unit.
 2. The engine of claim 1, wherein: each fan blade has a leading edge and a trailing edge; a splitter is positioned along the flowpath through the engine and having a leading rim separates a core branch of the flowpath from a bypass branch of the flowpath; an inboard boundary of the core branch of the flowpath has a radius R_(II) at an axial position of the splitter rim; the inboard boundary of the core branch of the flowpath has a radius R_(I) at a leading stage of blades of the first compressor; and a ratio of an axial length L₁₀ between the splitter rim and the leading stage of blades of the first compressor at the inboard boundary of the core branch of the flowpath to the radius R_(II) is less than 1.2.
 3. The engine of claim 1, wherein: each fan blade has a leading edge and a trailing edge; a splitter is positioned along the flowpath through the engine and having a leading rim separates a core branch of the flowpath from a bypass branch of the flowpath; an inboard boundary of the core branch of the flowpath has a radius R_(II) at an axial position of the splitter rim; the inboard boundary of the core branch of the flowpath has a radius R_(I) at a leading stage of blades of the first compressor; and a ratio of the radius R_(I) to the radius R_(II) is greater than 0.50 and is less than 1.0.
 4. The engine of claim 3, wherein: said ratio of the radius R_(I) to the radius R_(II) is 0.55-1.0.
 5. The engine of claim 4, wherein: the fan blades are non-variable.
 6. The engine of claim 5, further comprising: a variable fan nozzle.
 7. The engine of claim 6, wherein: the forward hub extends forward from a disk of the first compressor.
 8. The engine of claim 7, wherein: the forward hub extends forward from a bore of the disk of the first compressor.
 9. The engine of claim 1, wherein: a separation of a transverse centerplane of the first bearing and a transverse centerplane of the second bearing is less than a radius (R_(B)) of the first bearing.
 10. The engine of claim 1, wherein: a first seal, seals the first bearing and a second seal seals the second bearing to isolate a transmission compartment ahead of the first bearing relative to the longitudinal axis and the second bearing from a region behind the first bearing and the second bearing relative to the longitudinal axis.
 11. The engine of claim 1, wherein: the transmission comprises: a sun gear mounted to rotate with the first shaft; said ring gear; a plurality of intermediate gears between the sun gear and the ring gear; and a carrier holding the intermediate gears.
 12. The engine of claim 11, wherein: a third of said main bearings is a thrust bearing engaging the first spool shaft; and a fourth of said main bearings is a non-thrust roller bearing engaging an aft end of the first spool shaft.
 13. The engine of claim 1, wherein: said second pressure turbine is a high pressure turbine; and said first compressor is a high pressure compressor.
 14. The engine of claim 13, wherein: the first pressure turbine has three to five blade stages.
 15. The engine of claim 14, wherein: the second spool shaft engages at least two of said main bearings, at least one of which is a thrust bearing.
 16. The engine of claim 15, wherein the ratio of R_(H) to R_(T) is less than 0.30.
 17. A turbofan engine comprising: a fan having a plurality of blades rotatable about a longitudinal axis; a transmission (configured to drive the fan; and a first spool and a second spool, wherein: the fan blades have a peak tip radius R_(T); the fan blades have an inboard leading edge radius R_(H) at an inboard boundary of a flowpath; a ratio of R_(H) to R_(T) is less than 0.40; said first spool comprises: a first pressure turbine; and a first shaft coupling the first pressure turbine to the transmission; said second spool comprises; a second pressure turbine; a first compressor; and a second spool shaft coupling the second pressure turbine to the first compressor; the engine comprises a plurality of main bearings wherein: a first of said main bearings engages a static support and a forward hub of the second spool; and a second of said main bearings engages the first shaft and the forward hub of the second spool; and the first bearing and the second bearing are behind the transmission relative to the longitudinal axis.
 18. The engine of claim 17, wherein: the transmission comprises: a sun gear mounted to rotate with the first shaft; a ring gear mounted to rotate with the fan; a plurality of intermediate gears between the sun gear and the ring gear; and a carrier holding the intermediate gears.
 19. The engine of claim 17, wherein: the fan is a single-stage fan.
 20. The engine of claim 17, wherein the ratio of R_(H) to R_(T) is less than 0.30.
 21. A turbofan engine comprising: a fan having a plurality of blades; a transmission configured to drive the fan; and a first spool and a second spool, wherein: the fan blades have a peak tip radius R_(T); the fan blades have an inboard leading edge radius R_(H) at an inboard boundary of a flowpath; a ratio of R_(H) to R_(T) is less than 0.40; said first spool comprises: a first pressure turbine; and a first spool shaft coupling the first pressure turbine to the transmission; said second spool comprises; a second pressure turbine; a first compressor; and a second spool shaft coupling the second pressure turbine to the first compressor; the first compressor has a rear hub engaging a bearing; said bearing engages the first shaft; and another bearing engages the first compressor and the first shaft.
 22. The engine of claim 21, wherein: the transmission comprises: a sun gear mounted to rotate with the first shaft; a ring gear mounted to rotate with the fan; a plurality of intermediate gears between the sun gear and the ring gear; and a carrier holding the intermediate gears.
 23. The engine of claim 21, wherein: the fan is a single-stage fan.
 24. The engine of claim 23, wherein the ratio of RH to RT is less than 0.30. 